This invention relates to a thermal expansion valve for use in a heat pump system, and, in particular, to such a thermal expansion valve having an internal check valve.
The operational features of a heat pump system are well known in the art. In general, such systems include a compressor which forces refrigerant to a four way reversing valve. In the cooling cycle, the refrigerant flows from the reversing valve to an outdoor coil (i.e., a condenser), through an expansion valve to an indoor coil (i.e., an evaporator), and back to the compressor by way of the reversing valve. Typically, thermal expansion valves have a relatively small orifice through which the refrigerant entering the cooling coil must flow thus causing an adiabatic expansion of the refrigerant.
Because of the relatively small diameter orfice, thermal expansion valves operate only in one direction. In reverse flow conditions, an attempt to force the refrigerant through the expansion orfice would unduly restrict refrigerant flow. Accordingly, prior art heat pumps were provided with a by-pass around the expansion valves with the by-pass having an external check valve so as to permit flow through the by-pass in only one direction. This separate check valve/by-pass line usually required a field installer to provide two tees in the line on either side of the thermal expansion valve with the check valve installed parallel to the thermal expansion valve. The need for field installation and multiple joints inherent in the use of such external check valves makes the use of such external check valves expensive. It also increases the possibility for leaks and makes infield service checks more difficult and more expensive.
Expansion valves having built-in check valves are known. These overcame the problems of valves with external valves, but they have problems of their own. In one such valve, as shown in U.S. Pat. No. 4,964,567 to Heffner et al, the integral check valve is a flapper check valve. Flapper valves are typically gravity dependent. If mounted in an upright or sideways position, fluid flow is required to keep the valve closed. When mounted upright, gravity acts against the fluid pressure to keep the valve open. Thus, when the heat pump compressor operates the system under low pressure, there may be more pressure pushing the valve open than pushing it closed, and the flapper cannot be maintained closed. This problem is especially acute when fluid pressure is low. Because the check valve cannot be kept closed, it is difficult to control expansion of the liquid through the expansion valve. Further when the valve is used under high pressure, there is a time lag between the start of high pressure flow through the expansion valve and the closing of the flapper valve. During this time period, the valve remains open, and refrigerant can flow into the by-pass tube. Control of the expansion valve is therefore also made difficult. The by-pass tube of this valve is external to the valve. It thus includes auxiliary ports which provide for extra joints which may leak.
Another expansion valve with a built-in check valve is shown in U.S. Pat. No. 4,852,364 to Seener et al. Seener uses a spring biased stem which passes through an adjustable partition member, a slidable cup shaped check valve element, and a guide member to communicate with a follower member of a diaphragm valve. The check valve element is slidable on the guide member. The cup-shaped control valve element has inlet apertures on its sidewalls and a control valve port on its bottom or end wall. Whether the valve element operates as an expansion valve or by-pass depends on the valve element's position on the guide member and its position in relation to a tapered portion of the stem which engages the control valve port. This tapered portion of the stem forms a check valve element. The construction of this valve is both complicated and expensive. Because there are so many parts which slide against each other, the parts must be machined very precisely, thus increasing the cost of production. Further, as the check valve element is dependent upon fluid flow to move it into the by-pass position or into the expansion valve position, the same lag times may be present as are present in the flapper check valve. Thus, this valve may also have problems with control of the superheat during this lag time.
A thermal expansion valve having an internal by-pass is shown in U.S. Pat. No. 3,699,778 to Orth. This expansion valve does not include a check valve. Rather it has complex valve means including an expansion valve member which seats against an expansion port and an internal chamber in this valve member. The internal chamber has equalization ports which are in communication with the valve inlet when the ports are opened The equalization ports are opened and closed by a collar which is connected to the diaphragm by push pins. The outlet of the expansion valve is in communication with a chamber directly beneath the diaphragm. When the compressor is in operation, the push pins push down on the collar to open the expansion port and close the equalization ports. When the compressor shuts down, the evaporator warms up and the forces across the diaphragm tend to balance. The pressure within the valve's internal chamber forces the collar upward opening the equalization ports, thereby allowing reverse flow through the internal chamber. As in the Seener et al expansion valve, there are many parts which will slide against each other requiring precise machining, increasing the cost of production.
Another expansion valve with an internal by-pass is shown in U.S. Pat. No. 3,252,297 to Leimbach et al. This valve includes a flow path having an inlet and an outlet. A slidable tubular member is received in the flowpath. The tubular member is smaller in diameter than the flowpath and thus defines two flowpaths. The inner circular flow path defines the expansion valve flow path and has an expansion port. The outer annular flow path defines the by-pass flow path and has a by-pass port. This is thus a complex valve and requires precise machining. It is complicated and expensive to produce.